Computers with built-in dial-up modems often connect to networks, such as the Internet, via a wired connection. One example of a wired connection is a two-wire telephone cord that connects a personal computer (PC) modem port to a wall jack. There, the two-wire telephone cord meets a twisted pair telephone line from the telephone company's central office (referred to as the “local loop”).
There are numerous limitations associated with the use of wired connections. For example, computers that require wired connections have limited portability, since their operation is dependent upon proximity to a nearby telephone jack or outlet. Wireless links therefore have been developed to reduce the need for wired connections. Generally speaking, a wireless link comprises a way of transmitting data without the use of wires.
One existing type of wireless link makes use of the IEEE 802.11b standard. One product of this type includes a base unit and one or more wireless circuit cards that can be mounted in portable computers. The IEEE standard makes it possible to achieve data rates up to 11 Mbit/s and to handle numerous simultaneous users in an Ethernet Local Area Network (LAN). The digital modulation format is either Frequency Hopping Spread Spectrum (FHSS) or Direct Sequence Spread Spectrum (DSSS) operating in the 2.4 GHz ISM (Industrial, Scientific, Medical) radio band.
Since very high data rates can be transmitted, the foregoing product is mainly used for distribution of signals from cable and DSL modems and for networking computers. The base unit in the product also contains a dial-up modem that can be connected to a telephone jack. The 56 kbit/s binary data stream associated with the dial-up modem, together with additional control bits, are converted into the 802.11b format and sent over the 2.4 GHz radio channel to one or more laptop computers. If only used to transmit a 56 kbit/s signal, this type of data transmission method can be rather expensive.
Other types of wireless modems exist to transmit a dial-up modem signal over a short radio path from a telephone wall jack to a single portable computer. These modems digitize an incoming V.34 or V.90 signal via an A/D (analog-to-digital) converter and transmit the resulting digital stream over a radio channel using QPSK (Quadrature Phase-Shift Keying) modulation. Since a V.90 signal has to be transmitted with very high fidelity, a high-bit (e.g., ten to twelve bit) A/D converter is needed on the transmitting side and a high-bit D/A (digital-to-analog) converter is needed on the receiving side.
The foregoing configuration results in transmission of high bit rates over a radio channel, requiring RF (radio frequency) bandwidths of about 200 kHz. Such bandwidth is much larger than the 30 kHz needed for analog FM cordless telephones or the 100 kHz needed for some digital cordless telephones. Excessive bandwidth requirements make modems that use the foregoing configuration less attractive.
The distance from an Internet user to a central office of a telephone company, where four-wire ISP (Internet Service Provider) equipment connects, can vary from a few hundred feet to typically about three miles. This makes the telephone interface difficult to match precisely, sometimes resulting in substantial echoes in the system. Most “wired” modems are therefore equipped with a modern echo canceller, which can remove much of the echoes.
In a wireless system, echoes sometimes surpass the data signals and can saturate the radio channel. A saturated or over-driven circuit causes nonlinear distortions in the data signal which render the echo canceller in the modems ineffective.
Heretofore, an echo canceller was included in the base unit of a wireless modem to eliminate echoes before they could saturate the RF channel. Since modern echo cancellers are digital, this configuration requires conversion of signals into digital and later conversion back into analog. The A/D and D/A conversions effectively introduce an added complication to the data channel and, unless very high-bit converters are used, slow down the V.90 speed to that of V.32 or V.34.
Signal levels, due to the distance variations mentioned above, can vary over 30 dB in a normal dial-up modem. This is not an issue during wired mode operation, since such a modem can handle the variations. However, signal level variation can become an issue in a wireless system. If the signal is reduced by 30 dB at the input of a frequency shift keying (FSK) radio channel, the frequency deviation will be reduced accordingly, reducing the signal-to-noise ratio by 30 dB and rendering the system inoperable. A standard way of handling this is to introduce an AGC (automatic gain control) or ALC (automatic level control) unit to make the input level constant. However, a modern modem sends out tones of different levels in the beginning of communication, called a “handshake”, to calibrate the modem circuits to account for the distance and echo variations. A conventional AGC/ALC circuit would interfere with this process, leaving the system unstable.